Git commit message syntax highlighting in Vim

Writing git commit messages should be done inside your
editor because it allows you to add a lengthier commit
description, as opposed to git commit -m which just
puts everything as the first line.

Well if your editor of choice is Vim, this is what
happens after you enter git commit.

Notice how first 50 chars of a commit message are
highlighted in red. This is used to notify you about
the length of the first line of a commit message so you
know when it's too lengthy. For Vim, 50 chars is the
suggested size of the first line of commit message.

Well this isn't a good default for me, since the only
thing that's worrying me is the truncation of the first
line of commit message when my code ends up on Github.
I'm only interested if the first line of my commit
message is short enough to be displayed in full
on Github.

If that's not the case I'll end up with something
like this.

Yuck.

Preventing this is easy. Just make Vim highlight the first
72 chars in red, which is the maximum number of characters
Github allows in the first line.

In order to do so, alter Vim's syntax rules for git commit
messages, by editing ~/.vim/after/syntax/gitcommit.vim
like this.